Process for cracking cycloolefins

ABSTRACT

Processes are described for the production of alpha, omegaalkadienes by subjecting cycloalkenes to thermal cracking in the presence of an ammonia or amine regulator. The presence of such regulator is shown to inhibit secondary reactions which produce byproducts having boiling points close to that of the desired alpha, omega-alkadiene, thus facilitating the recovery of highpurity diene by distillation.

United States Patent [72] Inventors [2i Appl. No. [22] Filed [45] Patented [73] Assignee [54] PROCESS FOR CRACKING CYCLOOLEFINS 14 Claims, No Drawings [52] US. Cl 260/680 C, 260/681 [51] Int. Cl C07c 11/12 [50] Field of Search 260/680, 681, 666 B, 680 C [56] References Cited UNITED STATES PATENTS 2,400,409 5/1946 Hale et al 260/68l 2,432,843 1 2/ 1 949 Whitman 3,287,436 li/l966 Ozero 260/680 3,366,703 l/1968 Frech 260/680 3,529,032 9/1970 Frech 260/680 OTHER REFERENCES Popov et al., Chemical Abstracts, Vol. 45, (195i), page 2,88l.

Primary Examiner-Paul Coughlan, Jr. AitorneyJ. Richard Geaman PROCESS FOR CRACKING CYCLOOLEFINS BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to the production of alpha, omega-alkadienes by the pyrolysis of cycloalkenes. More specifically, it relates to a process for inhibiting the formation of secondary reaction products having boiling points close to that of the desired alpha, omega-alkadiene by conducting the pyrolysis in the presence of an amine or ammonia regulator.

2. Description of the Prior Art It is well known in the prior art to subject a cycloalkene to an elevated temperature sufficient to effect isomerization or molecular degradation and to recover a wide variety of useful products having the same or lower molecular weight. Such prior art thermal cracking or pyrolysis operations generally are carried out simply by heating the cycloalkene, either alone or in the presence of an inert carrier, for a controlled period of time at cracking temperature; i.e. at a temperature sufficient to effect molecular rearrangement of that particular cycloalkene. It is also well known that product distribution can be adjusted within broad limits by controlling both the cracking temperature and residence time. For example, under mild conditions, such as the use of low cracking temperatures with moderate contact times or moderate cracking temperatures with short contact times, the converted cycloalkene is found to be largely alpha, omega-alkadiene, along with small quantities of low molecular weight degradation products. In the case of large ring cycloalkenes having from 7 to 12 carbocyclic carbon atoms, this alkadiene, which is an isomer of the cycloalkene feedstock is believed to be the principal product of the primary reaction, the opening of the carbocyclic ring. Using the somewhat more stable fiveand six-ring member cycloalkenes, alpha, omega-alkadienes may also be recovered, but these are generally lower molecular weight materials than the feedstock and are believed to be secondary products resulting from cleavage of a carbon-to-carbon bond in the primary isomerization product. Exposure to more severe conditions, such as by raising the temperature or increasing the residence time, favors the formation of molecular degradation products at the expense of the primary isomerization products. By increasing the severity of the cracking, it is possible to produce a product that is first largely diene, then ethylene and propylene and finally methane and coke. In order to minimize the formation of these molecular degradation products, as well as products of the reaction of these fragments with the feedstock or isomerization products, the cracking conditions employed in the production of alpha, omega-alkadienes are generally somewhat milder than those which would result in maximum alkadiene yield based on the total cycloalkene feed. By using such very mild conditions, cycloalkene conversion is reduced, but alkadiene selectivity is increased, i.e. the proportion of alkadiene in the converted product is increased. In spite of this increased selectivity, it has not been possible to completely avoid the formation of molecular degradation products which, under these mild conditions, readily recombine to form impurities which have boiling points very close to that of the desired alkadiene and which, therefore, are exceedingly difficult to remove by simple fractional distillation.

SUMMARY It is an object of this invention to provide a process for thermally cracking cycloalkenes to produce alpha, omega-alkadienes of high purity. It is a further object of this invention to provide a method for inhibiting secondary reactions during such cracking operations which produce byproducts having boiling points close to that of the desired alpha, omega-alkadiene. A specific object of this invention is to provide a process for thermally cracking a large ring cycloalkene to produce an alpha, omega-alkadiene having the same number of carbon atoms, which can readily be recovered in high purity by fractional distillation without the necessity of employing complex and expensive distillation columns. It has now been found that these objects and other features of advantage, which will be apparent from a consideration of the following detailed process description, can be achieved by operation in accordance with this invention. Broadly our invention is the discovery of an improved process for preparing an alpha, omega-alkadiene comprising pyrolyzing a cycloalkene in the presence of an amine or ammonia regulator.

The use of any quantity of such re gulator has been found to inhibit secondary reactions which produce byproducts having boiling points close to that of the alpha, omega-alkadiene, thus facilitating the recovery of high-purity diene in simple fractional distillation equipment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The improved process of this invention can be carried out simply by heating a vapor phase mixture of the cycloalkene and regulator at or above the minimum cracking temperature for that particular cycloalkene. As in the case of the prior art processes, this can be done in either batch or continuous manner, with the latter being greatly preferred for commercial operation. In a convenient method for conducting such continuous operation, the cycloalkene and regulator (and inert diluent, if employed) are mixed in the vapor phase at a temperature below the minimum cracking temperature of the cycloalkene, and the vapor phase mixture is immediately introduced into a heated tubular reactor at a rate sufiicient to maintain the desired residence time. Efi'luent from the reactor is rapidly cooled and condensed, the gases are vented and the alpha, omega-diene recovered by fractional distillation. Since the cycloalkene, as well as the isomerization and molecular degradation products are readily flammable, oxygen or other oxidizing agents should be essentially excluded during at least the high-temperature portions of the operation.

The pyrolysis may be conducted under normal, sub or super atmospheric pressures. It is generally desirable to maintain the absolute pressure of the cycloalkene at less than about 15 p.s.i. and preferably at less than about 5 p.s.i. This may conveniently be accomplished by diluting the cycloalkene feed with an inert carrier, such as water, nitrogen, an inert gas or an ethylenically saturated hydrocarbon. The amount of diluent which is employed is not critical and can, therefore, be varied over a very broad range. When using the preferred diluent, water, one may advantageously employ up to or more mols of water per mol of cyclooctene. It is, however, generally preferred that the mol ratio of water to cycloalkene be in the range of from about 2:1 to about 20: l.

The minimum temperature at which cracking will occur varies somewhat depending upon the particular cycloalkene employed. While some degree of cracking will generally occur at temperatures as low as about 750 F., and it is possible to operate without exceeding this figure, it is usually more satisfactory to rapidly heat the cycloalkene to a maximum pyrolysis temperature in the range of from about l,000to about 1,500 F. When employing cycloalkenes containing from about eight to about 10 carbocyclic carbon atoms, outstanding results can be obtained when this maximum temperature is within the range of from about l,lO0 to about l,250 F.

The period of time during which the cycloalkene is subjected to cracking temperatures may be varied over a wide range and the optimum period is largely dependent upon the maximum temperature to which it is exposed and the rate of heating and cooling. For example, while residence times of from less than 0.01 second to 30 seconds or more are operable, better results are usually obtained with the shorter contact periods, particularly when the maximum pyrolysis temperature is in excess of l,l00 F. Since alpha, omega-alkadiene selectivity is inversely related to cycloalkene conversion, it is desirable to adjust the residence time at any suitable pyrolysis temperature to effect a cycloalkene conversion of less than about 70 percent. Conversions in the range of about 10 percent to about 65 percent are greatly preferred, with outstandlUlOlIIZ ing results being obtained at from about 25 percent to about 50 percent conversion levels.

The cycloalkenes which are preferred for use in the process of this invention are those which contain from about seven to about 12 carbon atoms, all of which are present in a single-.

carbocyclic ring; i.e. the homologous series from cycloheptene through cyclododecene. The use of cyclooctene, cyclononene or cyclodecene represents an especially preferred embodiment of this invention.

While alpha, omega-alkadienes are preferably produced from a feedstock containing a preformed cycloalkene, one may also employ a feedstock, which under pyrolysis conditions, decomposes to liberate such unsaturated hydrocarbon. Thermally decomposable materials of this type include the cycloalkenols and cycloalkyl esters, such as cyclooctanol, cyclononanol and cyclodecanol and carboxylic acid esters thereof.

As indicated above, the use of gaseous ammonia or a vaporized substituted ammonia is an essential feature of this invention. While substituted ammonia compounds, such as methylamine, dimethylamine, ethylamine, ethylene diamine and hydrazine, which have boiling points below the pyrolysis temperature are suitable, the use of gaseous ammonia is greatly preferred. The beneficial effect of the use of such regulator becomes evident when as little as a trace quantity is present. Generally, however, for maximum advantage it is preferred that at least about 0.01 mol of the regulator be employed per mol of the cycloalkene. The use of at least from about 0.02 to about 0.1 mol of regulator per mol of cycloalkene is especially preferred. When the cycloalkene is produced in situ by the thermal decomposition of a cycloalkyl ester, it is often desirable to augment such quantity of regulator with sufficient additional ammonia or amine to neutralize the liberated carboxylic acid. While there is no critical upper limit on the quantity of regulator that may be employed, there is no significant advantage in greatly exceeding the preferred concentration described above.

The following comparative examples illustrate the beneficial effect of employing the novel process of this invention.

EXAMPLE [A Cyclooctene is vaporized and mixed with steam to provide a feed mixture containing 9.4 mol percent cyclooctene and 90.6 mol percent water. This mixture is introduced at the rate of 325 pounds per hour into a stainless steel tubular reactor having an inside diameter of 1 inch and a length of 528 inches. The reactor temperature, which increases from about l,l at the inlet to about l,200 F. at the outlet, is maintained by multiple electric heaters. The reactor effluent is cooled rapidly to about 100 F. and uncondensed gases are vented. After 4 hours of operation, the gas-free reactor effluent stream is sampled. The liquid aqueous and hydrocarbon phases of this sample are separated and the latter, in which the cyclooctene conversion is about 45 percent, is distilled. The fraction boiling between 242 and 249 F. is analyzed by vapor phase chromatography and found to contain 88.5 weight-percent 7-octadiene.

EXAMPLE [B The procedure of example IA is repeated except that ammonia is mixed with the cyclooctene and steam to provide a feed mixture containing 9.4 mol percent cyclooctene, 90.2 mol percent water and 0.4 mol percent ammonia. After 4 hours of operation with this feed, a sample of the liquid hydrocarbon reactor effluent boiling between 242 and 249 F. is recovered and analyzed as in example 1A and found to contain 97.2 weight-percent l, 7-octadiene.

EXAMPLE ll The procedures of examples IA and IB are repeated except that cyclononene is substituted for cyclooctene and the sampled fraction of the liquid hydrocarbon reactor effluent has a boiling range of 287 to 294 F. The use of ammonia results in a decrease in the non l, 8-nonadiene content of this fraction of more than 50 percent.

EXAMPLE Ill The procedures of examples IA and 18 are repeated except that cyclodecene is substituted for cyclooctene. the feed rate is reduced to 300 pounds per hour and the sampled fraction of the liquid hydrocarbon reactor effluent has a boiling range of 326 to 333 F. The use of ammonia greatly reduces the impurity level of this 1, 9-decadiene fraction.

EXAMPLE IV A stream of nitrogen is bubbled at a rate of 600 milliliters per minute through liquid cyclooctyl acetate in a flask which is heated over a steam bath. The vapor space in the flask is vented to a stainless steel tubular reactor having an inside diameter of one-half inch and a length of 18 inches. The reactor, which is packed with /4-inch diameter ceramic balls, is maintained at about l,lOO F. by electric heaters. The reactor effluent is cooled rapidly to about 100 F. and uncondensed gases are vented. After 1 hour of operation, the entire liquid effluent is mixed with a large volume of water, agitated and then allowed to settle. The separated hydrocarbon phase is added to a large volume of dilute aqueous sodium bicarbonate and again agitated and allowed to settle. The neutralized hydrocarbon phase is then removed and distilled. Vapor phase chromatographic analysis of the fraction boiling between 242 and 249 F. shows less than weight-percent l, 7octadiene.

EXAMPLE V The procedure of example IV is repeated except that ammonia is substituted for nitrogen. Vapor phase chromatographic analysis of the effluent hydrocarbon fraction boiling between 242 and 249 F. shows more than weight-percent l, 7-octadiene.

lt will, of course, be understood that various changes may be made in the preferred embodiments of this invention illustrated above without departing from the spirit and scope of the invention as defined in the following claims.

We claim:

1. Process for preparing an alpha, omega-alkadiene comprising thermally cracking a cycloalkene in the presence of an amine or ammonia regulator.

2. The process of claim 1, wherein said regulator is ammonia.

3. The process of claim 1, wherein said cycloalkene contains from seven to 12 carbocyclic carbon atoms.

4. The process of claim 3. wherein said cycloalkene is formed in situ by the thermal cleavage of a cycloalkanol or cycloalkyl ester.

5. The process of claim 3, wherein said cycloalkene is cyclooctene.

6. The process of claim 1, wherein said cycloalkene is admixed with an inert gaseous carrier.

7. The process of claim 6, wherein said carrier is water.

8. The process of claim 1, wherein at least about 0.01 mol of said regulator is present per mol of said cycloalkene.

9. The process of claim 1, wherein from about 0.02 to about 0. 1 mol of ammonia is present per mol of said cycloalkene.

10. Process for preparing an alpha, omega-alkadiene comprising thermally isomerizing a gaseous mixture of a cycloalkene containing from about eight to about 10 carbocyclic carbon atoms, water and ammonia at a temperature above about 750 F. for a period of time sufficient to convert from about l0 to about 65 percent of said cycloalkene and recovering said alpha, omega-alkadiene.

11. The process of claim 10, wherein said cycloalkene is cyclooctene: and said alpha, omega-alkadiene is l, 7-octadiene.

12. The process of claim 11, wherein said mixture is heated to a maximum temperature range of from about 1.100 to about l,250 F.

13. The process of claim 12, wherein said mixture contains from about 0.02 to about 0.1 mol of ammonia and from about 2 to about mols of water per mol of cyclooctene.

14. The process of claim 13, wherein cyclooctene conversion is from about to about 50 percent. 5 

2. The process of claim 1, wherein said regulator is ammonia.
 3. The process of claim 1, wherein said cycloalkene contains from seven to 12 carbocyclic carbon atoms.
 4. The process of clAim 3, wherein said cycloalkene is formed in situ by the thermal cleavage of a cycloalkanol or cycloalkyl ester.
 5. The process of claim 3, wherein said cycloalkene is cyclooctene.
 6. The process of claim 1, wherein said cycloalkene is admixed with an inert gaseous carrier.
 7. The process of claim 6, wherein said carrier is water.
 8. The process of claim 1, wherein at least about 0.01 mol of said regulator is present per mol of said cycloalkene.
 9. The process of claim 1, wherein from about 0.02 to about 0.1 mol of ammonia is present per mol of said cycloalkene.
 10. Process for preparing an alpha, omega-alkadiene comprising thermally isomerizing a gaseous mixture of a cycloalkene containing from about eight to about 10 carbocyclic carbon atoms, water and ammonia at a temperature above about 750* F. for a period of time sufficient to convert from about 10 to about 65 percent of said cycloalkene and recovering said alpha, omega-alkadiene.
 11. The process of claim 10, wherein said cycloalkene is cyclooctene and said alpha, omega-alkadiene is 1, 7-octadiene.
 12. The process of claim 11, wherein said mixture is heated to a maximum temperature range of from about 1,100* to about 1,250* F.
 13. The process of claim 12, wherein said mixture contains from about 0.02 to about 0.1 mol of ammonia and from about 2 to about 20 mols of water per mol of cyclooctene.
 14. The process of claim 13, wherein cyclooctene conversion is from about 25 to about 50 percent. 